1. Field of the Invention
The present invention relates to a light emitting device and a manufacturing method thereof, and a light emitting display and a manufacturing method thereof.
2. Description of the Related Art
In general, a light emitting device is a device that can actively emit light from an emitting part formed between two electrodes on a substrate. A light emitting device can be classified by driving method into active matrix type, in which the emitting part is driven by a thin film transistor (hereinafter, “TFT”), and passive matrix type, in which the emitting unit is driven by other means. A light emitting device can further be classified by direction of emission into bottom-emission type, in which light is emitted toward the substrate, and top-emission type, in which light is emitted toward opposite direction of the substrate. On the other hand, a light emitting device can be classified by the emitting materials used into organic light emitting type that comprises an organic luminescence layer, and an inorganic type that comprises an inorganic luminescence layer.
For example, pixel part of a top-emission type active matrix organic light emitting diode (hereinafter, AMOLED) comprises switching thin film transistors (hereinafter, TFTs) for switching the pixels, driving transistors, storage capacitors, anodes, organic material layers, and common electrodes (cathodes).
FIG. 1 is a sectional view of a pixel on the axis of a thin film driving transistor in a conventional organic light emitting device.
Referring to FIG. 1, a conventional organic light emitting device comprised an organic light emitting layer 12 to be formed in a plurality of pixels that are defined by cross-areas of pixel electrodes 8, cathodes (metal electrode) 15, and transparent cathodes 16, each of which are formed in plurality on a glass substrate 1; a TFT A formed on the glass substrate 1, while drain terminal thereof is electrically connected to the pixel electrodes 8; a hole injection layer (hereinafter, hole injection layer) 10 as well as a hole transfer layer (hereinafter, hole transport layer) 11, both formed in layers between the pixel electrode 8 and the organic light emitting layer 12; and an electron transfer layer (hereinafter, electron transport layer) 13 as well as an electron injection layer (hereinafter, electron injection layer) 14, both formed in layers between the organic light emitting layer 12 and a metal common electrode 15.
The TFT A comprised a semiconductor layer 2 which is formed on an area of glass substrate 1 and is consisted of source/drain areas 2a, 2b and channel area 2c; a gate insulation film 3 formed on the whole area of glass substrate 1 including a semiconductor layer 2; and a gate electrode 4 formed on the gate insulation film 3 over the channel area 2c. 
Here, boundary between the source/drain areas 2a, 2b and the channel area 2c was aligned to positive edge of the gate electrode 4.
In addition, an inter-layer insulation film 5 was formed on the TFT A for opening source area 2a and drain area 2b, to allow electrode lines 6 to be connected electrically to the source/drain areas 2a, 2b through openings of the inter-layer insulation film 5.
Further, a leveling insulation film 7 for opening the electrode lines 6 which is electrically connected to drain area 2b, was formed at front surface of the inter-layer insulation film 5 inclusive of the electrode lines 6.
On the leveling insulation film 7, a pixel electrode 8 was formed which is electrically connected to drain area 2b of the TFT A through openings of the leveling insulation film 7.
An insulation film 9 was formed for burying a part of pixel electrode 8 between the neighboring pixel electrodes 8.
An hole injection layer 10, a hole transport layer 11, an organic light emitting layer 12 of any one of R, G, B, an electron transport layer 13, and an electron injection layer 14 were formed successively on the pixel electrode 8.
The common electrodes 15, 16 are consisted of metal common electrode 15 and transparent common electrode 16, both formed on the electron injection layer 14, whereby a protective film 17 was formed on the transparent common electrode 16.
A description of the manufacturing process of a conventional light emitting device is given below making reference to FIG. 2.
FIGS. 2a to 2d show process of manufacturing a conventional light emitting device.
Referring to FIG. 2a, a semiconductor layer 2 to be used as an activation layer for TFT was formed on a glass substrate 1 utilizing a polycrystalline silicone, etc., and then, the semiconductor layer 2 is patterned in a manner that only the area designed for the TFT remains.
Then, a gate electrode 4 was formed by pattering conductive film of the gate electrode in a manner that the conductive film remains on an area of the patterned semiconductor layer 2, after a gate insulation film 3 and a conductive film of the gate electrode have been formed successively in layers.
The source/drain areas 2a, 2b of TFT were formed by injecting dopants such as B, P, etc. into semiconductor layer 2, and masking the same with gate electrode 4 successively, and then, heat-treating the same, whereby semiconductor 2 with no injected dopant was in channel area 2c. 
After that, an inter-layer insulation film 5 was formed at front surface, and contact holes were formed by selectively removing the inter-layer insulation film 5 and the gate insulation film 3, so that the source/drain areas 2a, 2b of the TFT were exposed.
Then, a first metal film having a thickness to sufficiently bury the contact holes was formed, and electrode lines 6 to be connected electrically to the source/drain areas 2a, 2b were formed by selectively removing the first metal film such that the metal film remained only at contact holes and adjacent areas thereof.
Successively, the front surface was leveled by forming a leveling insulation film 7 at front surface, and contact holes were formed by selectively removing the leveling insulation film 7 such that electrode lines 6 connected to drain area 2b were exposed. After that, a second metal film made of metals with high reflexivity and work function such as Cr, Al, Mo, Ag, Au, etc. was added to the front surface.
Here, a second metal film was formed also in contact holes so that the metal film was connected to electrode lines 6 at bottom of the contact holes.
Then, pixel anode or pixel electrode 8 to be connected to lower drain area 2b through electrode line 6 was formed by selectively removing the second metal film such that the same remained only at pixel parts.
Referring to FIG. 2b, an insulation film 9 was formed to cover a part of the pixel electrodes 8 between the neighboring pixel electrodes 8.
Referring to FIG. 2c, a hole injection layer 10 and a hole transport layer 11 were deposited to as common organic layers, organic light emitting layers 12 for R, G, B are formed using shadow masks, and then, organic layers such as electron transport layer 13 and electron injection layer 14 were formed successively.
Referring to FIG. 2d, after the organic layers (10 to 14) have been formed, a metal common electrode 15 was formed thereon, whereby the metal common electrode 15 was made by addition of Al in a thickness of several nm and a successive addition of Ag in a thickness of several nm to several tens of nm, or of other metals such as MgxAg1-x, etc. in a thickness of several nm to several tens of nm.
In addition, a transparent common electrode 16 was formed on the metal common electrode 15 using a transparent conductive material such as ITO, IZO, etc.
Finally, a protective film 17 for protection of the organic layers (10 to 14) from oxygen, humidity, etc. was formed and then installed using a sealant and a transparent substrate, with which step a top-emission type active matrix organic light emitting device was completed.
In a top-emission type active matrix organic light emitting device as above, light generated by recombination of holes and electrons at organic light emitting layers was emitted through metal cathode 5, in contrast to a bottom-emission type organic light emitting device, wherein light was emitted from bottom of the substrate. Accordingly, thickness of a metal film to be used as a metal common electrode 15 in such a bottom-emission type organic light emitting device could not be sufficiently thick and was limited generally to several nm to several tens of nm to secure a desirable transmission rate.
However, as a large amount of current flows continuously through the metal common electrode 15 in an organic light emitting device, a short by heat or oxidization could occur if the metal common electrode 15 was not sufficiently thick.
In particular, in cases where Ag was used for the metal common electrode 15, lumping could occur due to migration of Ag atoms, leading to reduced lifetime and decreasing of reliability of the product.
On the other hand, if thickness of the metal common electrode 15 was increased to 10 nm˜15 nm, or even to 20 nm, to solve the above problems with lifetime shortening and reliability decrease, the transmission rate fell rapidly and the emitting efficiency was reduced substantially.
On the other hand, FIGS. 3 and 4 show an organic light emitting display which comprises a pad part formed with an organic light emitting device electrically connected to a wiring to drive the organic light emitting device.
FIG. 3 is a plane view of a conventional organic light emitting display, and FIG. 4 is a sectional view of the organic light emitting display shown in FIG. 3 taken along area X˜X′.
Referring to FIG. 3, the conventional organic light emitting device 300 was formed with a pixel circuit part 308 on which a plurality of pixels (not shown) are positioned, and a wiring part 306 electrically connected to the pad part 304 to drive the organic light emitting device.
In addition, a cathode electrode 310, which is a common electrode for applying voltages to a ground power source, was formed on the wiring part 306 electrically connected to the pad part 304.
More specifically, the conventional organic light emitting device 400 as shown in FIG. 4, was formed with a gate insulation film 403 for insulating a gate electrode on the substrate 302, and a inter-layer insulation film 405 for opening a source area (not shown) and a drain area (not shown). In addition, on the upper side of the inter-layer insulation film 405 was formed a leveling insulation film 407 for leveling the upper surface of the inter-layer insulation film 405, and the wiring part 306 was formed so as to apply voltages to the ground power source to electrically connect the inter-layer insulation film 405 to the pad part 304 through a contact hole P1 of the leveling insulation film 407.
In addition, the cathode electrode 310 being a common electrode was formed to be electrically connected to the wiring part 306 exposed through the contact hole P1, and to cover a hole injection layer 411, a hole transport layer 413, a light emitting layer 414, an electron transport layer 415, and an electron injection layer 417.
Finally, a protective film 419 for preventing moisture and oxygen from being penetrated was formed on the upper side of the cathode electrode 310.
In the conventional organic light emitting display 300, 400 thusly constructed, a great amount of current flew continuously through the cathode electrode 310 being a common electrode electrically connected to the wiring part 306 due to the characteristic of the organic light emitting device. At this time, there occurred a problem that in a case where a great amount of current flew continuously through the cathode electrode 310 formed thin, the cathode electrode 310 became short or oxidized due to heat generated by the current. Accordingly, there occurred a problem that the lifetime of organic light emitting device is shorted, the trustworthy of organic light emitting device is dropped.